Synthetic fibers, films and other molded articles utilize plastic materials which are light in weight, tough, and are capable of being mass-produced stably at a low cost, whereby having been providing a convenient satisfactory life which is so-called a plastic civilization. However, recent concerning on global environment raises a demand for the polymeric materials which can be degraded in environment. Among such materials, plastic materials capable of being degraded by microorganisms are especially attractive and expected to be used as environment-conscious materials as well as novel functional materials.
It is well known that aliphatic polyesters are biodegradable. Representatives of such polyesters are poly-3-hydroxybutyric acid (PHB) which is produced by micro-organisms and poly-.epsilon.-caprolactone (PCL) and polyglycolic acid (PGA) which are synthetic polymers.
Although the polyesters mainly consisting of PHB have excellent compatibility with the environment as well as physical properties and are produced in an industrial scale, their producibility is poor and they are of only limited use in view of the cost when used as the substitutes of widely-employed plastics such as polyethylene (See "Fibers and industry", Vol. 47, page 532 (1991)). On the other hand, PCL provides highly polymerized materials capable of being formed into fibers or films, but has poor heat resistance due to its melting point as low as 65.degree. C. or lower, resulting in impossibility of being applied widely (See "Polym. Sci. Technol.", Vol. 3, page 61 (1973)). PGA and glycolide-lactide (9:1) copolymer which are used practically in biocompatible sutures are non-biologically hydrolyzed and then metabolized and absorbed in vivo, but are expensive and have poor water resistance, thus being not suitable to be employed widely.
Aliphatic polyesters obtained by melt condensation polymerization of .alpha., .omega.-aliphatic diols and .alpha., .omega.-aliphatic dicarboxylic acids, such as polyethylene succinate (PES), polyethylene adipate (PEA) and polybutylene succinate (PBS), are well known, which can be produced at a low cost and are proven in in-soil tests to be capable of being biodegraded by microorganisms (See "Int. Biodetetn. Bull.", Vol. 11, page 127 (1975) and "Polym. Sci. Technol.", Vol. 3, page 61 (1973)). However, these polymers have poor thermal stability, and undergo concomitant degradation reaction upon the condensation polymerization. Accordingly, they usually have a molecular weights as low as 2,000 to 6,000 (reduced specific viscosity .eta.sp/c at 0.5 g/dl in chloroform determined at 30.degree. C. is 0.3 or lower), which are not suitable to be used in fibers or films. Among them, polyethylene succinate and polybutylene succinate have the melting points of 100.degree. C. or higher and are reported to have excellent biodegradability, but because of their poor thermal stabilities during polymerization the materials having molecular weights sufficient to be molded can not be obtained by ordinary methods. For example, in the reference cited above (Int. Biodetetn. Bull., Vo. 11, page 127 (1975)), a commercial low molecular polyethylene succinate is heated under reduced pressure to obtain a material capable of film-forming, but the molecular weight obtained is in the order of about 20,000, which failed to provide satisfactory functions. A polyethylene succinate having molecular weight of 39,600 obtained by condensation polymerization of succinic acid and ethylene glycol via stepwise removal of low molecular materials using methanol (See Die Makromolekulare Chemie, Vol 140, page 65 (1970)) has no sufficient strength in view of moldability, and is obtainable only by such complicated procedure, which gives very low yield of high molecular polyesters.
In another attempt, a polyethylene succinate and a polybutylene succinate having the molecular weights of 10,000 or higher are synthesized using titanium oxyacetyl acetonate or alkoxytitanium compound as the catalysts by a direct condensation polymerization method, but the molecular weights are not more than about 15,000, which could not provide moldable aliphatic polyesters (See JPA H5-70572, JPA H5-70566 and JPA H5-70574). In addition, this method suffered from the problem of polymer staining unless the catalysts are used only in minimum amounts.
Accordingly, it is attempted to increase the molecular weight of such aliphatic polyesters by the treatment with diisocyanates such as hexamethylene diisocyanate and tolylene diisocyanate (See "Polym. J.", Vol. 2, page 387 (1971) and JPA H4-189822). However, this method involves the complicated 2-step reaction process, slight reduction in crystallinity and melting point as well as formation of urethane bonds in the molecules which gives somewhat reduced biodegradability, although it gives the increased molecular weights.
Thus, the present invention provides the high molecular weight aliphatic polyesters based on polybutylene succinate and polyethylene succinate and the method of preparing the same, which eliminates the problems associated with the conventional methods, prevents staining and do not affect naturally-associated biodegradability, and gives the molecular weights sufficient to be utilized for molded articles.